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FAIR VALUE MEASUREMENTS, Assets and Liabilities Classified as level 3 (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Changes in fair value of assets and liabilities classified as level 3 [Roll Forward]  
Net derivative assets/(liabilities), Beginning balance $ (57,804)uil_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisNetAssetAndLiabilityValue
Unrealized gains and (losses), net (14,967)us-gaap_IncreaseDecreaseInRegulatoryAssetsAndLiabilities
Net derivative assets/(liabilities), Ending balance (72,771)uil_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisNetAssetAndLiabilityValue
Change in unrealized gains (losses), net relating to net derivative assets/(liabilities), still held as of March 31, 2015 $ (14,967)uil_FairValueNetAssetsAndLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss